Summary: Pneumococcal PSs have been covalently linked to carrier proteins by the reductive amination. In this method, the PSs were oxidized by periodate to introduce aldehyde groups. The degree of periodate oxidation is an important first step in the conjugation process. Thus, immunochemical characterization of periodate activated PSs can provide useful information for control tests of pneumococcal conjugate vaccines. The objective of the present study was quantitative analysis of the PS content in the monovalent and 7-valent pneumococcal conjugate vaccines using Light scattering rate nephelometry. The antibody binding reactions to periodate activated and then reduced PS were also examined, appling rate nephelometry for quantitative determination of PS content. Using this method, the individual types 4, 6B, 9V, 14, 18C, 19F and 23F PSs were found to range between 82.3 to 119% of the manufacturer's indicated values. Treatment of the conjugates with trypsin at 37 C resulted in increased PS content measurements for some types, including types 4 and 14. In addition, higher or lower levels of antigen-antibody reaction were introduced in periodate activated and then reduced PS for some serotypes compared to non-treated PS. Use of oxidized and reduced PS could provide an early indication of change in antigen-antibody binding and conjugate stability.